1. Field of the Invention
This invention relates generally to methods and apparatuses for braiding articles and more specifically to three-dimensional braiding of fibers useful inter alia as fiber reinforced structural preforms.
2. Description of the Related Art
Braiding apparatuses generally consist of a braiding surface upon which travel a plurality of yarn carrier members which dispense the braiding fibers. The braiding fibers generally intersect in an area near the article being braided, hereafter referred to as the braiding zone.
Various braiding surfaces have been developed, with the majority being a simple flat plane, as disclosed in U.S. Pat. No. 4,881,444. Flat surfaces have the disadvantage that extremely large surface areas may be needed to accommodate a moderate range of braiding angles. In addition, flat braiding surfaces cause difficulties in maintaining yarn tension, since carrier members at different braid angles require different length yarns. To address this difficulty, some braiding machines have curved braiding surfaces that attempt to maintain constant yam tension by maintaining the carrier members at a constant distance from the braiding zone.
Various braiding patterns are possible by manipulation of carrier member positions on the braiding surface. Many devices use a push/pull mechanism to change the carrier positions of entire rows or columns of carrier members, as disclosed in U.S. Pat. No. 4,885,973. Other devices use self-propelled carriers traveling in a fixed pattern determined by a preset track arrangement, as disclosed in U.S. Pat. No. 4,972,756.
Most braiding machines incorporate some features to maintain yarn tension and to rewind yarn. A common means to accomplish these goals is a coil spring or an electric motor with a friction coupling.